This application addresses methods and devices for measuring atmospheric parameters. More particularly, the invention relates to cost-minimized atmospheric sondes transmitting a signal containing atmospheric data and information from which the sonde position can be determined, and a set of receivers and processors for tracking the trajectory of the sonde as it moves through the atmosphere. Weather information can in turn be determined from the data transmitted by the sondes and by monitoring their paths over time.
In general, accurate weather prediction requires measurement of atmospheric parameters, called vertical profiles, in numerous volumes of the atmosphere. These profiles are frequently determined by a balloon- or parachute-borne instrument package, called a sonde, which moves through the atmosphere and transmits atmospheric data measured in situ back to a base station, which may be mobile, airborne or fixed. As the sonde rises or falls and drifts, the trajectory of the sonde is measured by the base station and the wind profile determined from that trajectory.
The present invention allows trajectory measurement using a sonde having only the minimal instrumentation required to acquire the relevant atmospheric data and transmit those data by radio, together with information needed to allow tracking of the sonde. By minimizing and simplifying the instrument suite and transmitter provided in each sonde, this invention has the potential to significantly decrease the cost of the sonde package; cost is important, since, given the difficulty of retrieving and reusing the sondes, they must be considered expendable.
The term xe2x80x9cradiosondexe2x80x9d implies a balloon-borne device to measure atmospheric parameters and transmit them by radio to a ground station. If the ground station tracks the device""s trajectory to measure the wind, the device is sometimes referred to as a xe2x80x9crawinsonde.xe2x80x9d The term xe2x80x9cdropwinsondexe2x80x9d is completely correct for sondes that are dropped and then tracked to measure winds aloft, but has generally been dropped in favor of the simpler term xe2x80x9cdropsondexe2x80x9d.
The first dropsondes measured pressure, temperature and humidity and sent these data back to an aircraft for processing into vertical profiles. Since the trajectory of the dropsonde was not measured, wind profiles could not be determined. Later, in the 1970""s, dropsondes were developed using the Omega radio-frequency navigation system to provide trajectory data, so that wind profiles could be determined. In the 1980""s, the capability to use the LORAN radio-frequency navigation system for tracking sondes increased the vertical resolution of the dropsonde wind profiles. The Omega system has been decommissioned and LORAN will similarly be decommissioned at some future time. Consequently a new generation of sonde designs has emerged that relies on the Global Positioning System (GPS) to provide very accurate sonde trajectory tracking, but with attendant increases in cost and complexity of sondes, which are still frequently unrecoverable.
A typical GPS-based approach to use of dropsondes for gathering wind information requires the sonde to have the capability either to receive and process GPS signals and to transmit position data to the base station, or, equivalently, the capability to receive and retransmit the GPS signal to the base station for processing there. The former approach is characterized by U.S. Pat. No. 5,347,285 to MacDoran et al. The latter approach, clearly preferred due to the inherently lower costs associated with processing the GPS signals for a number of sondes at a base station rather than in each sonde, is characterized by U.S. Pat. Nos. 5,420,592 to Johnson and U.S. Pat. No. 5,347,285 to Brown et al, differing mainly in the manner in which the relatively high-rate data streams are transmitted. U.S. Pat. No. 4,754,283 to Fowler uses GPS signals to determine the Doppler component of the signal from the drifting airborne sonde, from which the wind information is derived. In each of these prior art approaches the sonde must at minimum be equipped with a GPS receiving antenna and additional electronics. As noted, this adds expense to the frequently unrecoverable sondes.
Another set of references, U.S. Pat. No. 5,107,261 and U.S. Pat. No. 5,173,690 to Friedman et al, U.S. Pat. No. 5,053,784 to Hippelainem and U.S. Pat. No. 5,010,343 to Andersson, propose sonde tracking techniques that do not depend on GPS technology. However, the Friedman patents require additional sonde complexity to produce a separate ranging signal, and the Hippelainem method relies on a set of rotating ground station antennas. Finally, the Andersson device proposes switched sequential processing of signals received by individual antennas and further employs two additional reference antennas. Consequently, none of these references provides the minimal-cost sonde that would be preferred.
Because accurate knowledge of upper atmospheric conditions is becoming increasingly important in weather forecasting, communications, navigation and a host of other endeavors, the number and complexity of the sondes required has risen to a point where the viability and especially the affordability of a broad national or global network depends on economizing advances in sonde and sonde signal reception and processing. The present invention addresses this need.
A principal object of the invention is to reduce the cost of the sondes by minimizing the parts count to only those necessary, that is, to reduce cost by simplifying the measurement, processing and transmitting requirements placed on the sonde.
It is a corresponding object of the present invention to provide a method for accurately tracking the trajectories of the simplified sondes of the invention.
It is an additional object of the invention to provide a method for extracting sonde position from the data signal transmitted by the minimally-complex sondes according to the invention.
It is yet another object of the invention to provide a simple stacked antenna device adapted to simultaneously receive sonde signals at spaced locations to facilitate the above-cited method.
These and other objects and advantages of the present invention will be made clear from the descriptions and associated figures that follow.
Broadly, the invention comprises an inexpensive atmospheric sonde and base station weather data measurement system, and the methods of their use. The basic idea of the invention is to minimize the cost of the expendable sonde package by equipping it with only the minimum capability to measure the atmospheric data of interest and to transmit those data together with the minimum additional information necessary for sonde tracking back to a base station that may be fixed, mobile or airborne. Thus, the radio signal transmitted by the sonde is used for two different purposes, first as the carrier of the data, and second as a sounding signal to provide tracking information.
According to the invention, each sonde that is released (termed a xe2x80x9cdropsondexe2x80x9d if dropped from a plane, or a xe2x80x9cradiosondexe2x80x9d if lifted by a balloon) comprises a transmitter and various weather sensors. The binary data from each sensor is multiplexed together and the resulting bit stream is modulated by a binary pseudorandom sequence (PRS). The resulting signal is then used to control binary phase shift keying (BPSK) of the RF carrier. The PRS can be decoded at a receiver to identify the sonde, allowing it to be tracked over time, and the received signals are processed to determine the angle of arrival (AOA) of the signal at the base station. The sondes can be tracked from various fixed or moving ground stations, or airborne stations, as appropriate.
The sonde location is determined by a solution that relies on measurement of the carrier phase differences of the received signals at some number of receiving antennas spaced some little distance (i.e., meters) from one another. The difference in carrier phase as measured between pairs of antennas is sufficient to determine the signal""s angle of arrival, as measured with respect to a line extending between the two antennas. More specifically, according to an important aspect of the invention, the signal carrier phase difference as detected at the two antennas provides a precise but ambiguous indication of the difference in path length to the sonde; the difference in arrival times of the signal as measured at the two antennae, preferably determined by measuring the difference in time of reception of the transitions in the PRS-keyed signal, is used to remove this ambiguity.
According to an important aspect of the invention, the signal transmitted by a sonde is detected by closely-spaced antennas, and the difference in the identical signal as detected by the spaced antennas is used to derive the AOA of the signal at the antenna. Depending on which of several embodiments of the invention is being considered, this AOA may be the angle in three dimensional space or it may be the elevation angle at the site. In a first embodiment comprising a single base receiver site having at least three closely spaced receiver/antennas, this AOA in three dimensional space is used with the pressure-height value (the relationship between height above the earth""s surface and atmospheric pressure) and the base station coordinates to locate the sonde. The sonde location is tracked over time to determine its trajectory, from which the wind profile can them be determined. In a second embodiment, multiple time synchronized base stations are employed, each comprising two receiver/antennas that measure elevation angle of arrival (EAOA). These EAOA measurements, combined with the time of arrival (TOA) differences, allow determination of sonde trajectory and velocity without reliance on pressure measurements.
The employment of several such widely separated but time synchronized multi-receiver sites supports an unambiguous method for tracking sonde trajectories without the costly constraint of requiring in situ pressure measurement or employing on-board GPS-based location technology and transmission thereof by the sonde.